In the manufacture of a printed circuit board (hereinafter referred to as a "PCB") metallic copper may be laminated to either one or usually both sides of a suitable dielectric substrate, such as epoxy/fiberglass, paper impregnated with phenolic resins, or other synthetic materials. The metallic copper is usually oxidized on the "inner" side which is laminated to the substrate using heat or pre-cure lamination techniques. The outer surface of the copper clad is exposed to subsequent treatments utilized in the manufacture of the PCB, which include immersing in a number of solutions, such as cleaners, etchants, acid dips, activator and post-activator solutions. These pre-electroless plating treatment solutions become contaminated with copper dissolved from the copper clad PCB's which are immersed in them.
Soluble copper contaminants are of particular concern with respect to the activator solutions. These solutions typically contain palladium, or palladious ions, and tin, or divalent stannous ions, in an aqueous acid solution. The activator solution is utilized in the pre-treatment of the PCB substrate, in advance of contact with the desired electroless plating solution. However, the presence of soluble copper contaminants in an activator solution, which becomes evident from blue coloration in the solution, adversely affects the performance of the activator.
In commercial applications, frequent replenishment or replacement of the activator solution, which is expensive due to the palladium content, is required well before it would otherwise become exhausted. Furthermore, the presence of soluble copper contaminants is believed to catalyze air oxidation of divalent tin, which leads to decomposition of the activator solution. Finally, the presence of such copper contaminants can lead to "void plating" in holes on the PCB upon subsequent electroless metal plating.
The primary source of these soluble copper contaminants is the metallic copper clad laminated to the PCB substrate. Due to the acidic nature of several of the treatment solutions, particularly the acid dips and etchants, through which the copper clad PCB substrate is passed prior to treatment in the activator solution, metallic copper is dissolved. While this occurs in the activator solution itself, dissolved copper contaminants from these prior treatment solutions are also dragged into the activator solution as the PCB is moved from one treatment solution to the next.
Prior to the present invention, copper-contaminated activator solutions were discarded when the level of copper contaminants exceeded approximately 2,000 ppm. In addition to the expense of raw materials, particularly palladium, other disadvantages are suffered in terms of additional labor, waste treatment, and the time that the PCB manufacturing line is shut down.
The Electromotive Series indicates that the more noble metals, such as palladium, would be electrodeposited before copper. In the Electromotive Series, such as is set forth in the text, Modern Electroplating, by F. A. Lowenheim, page 776, 3rd ed. (1974), John Wiley & Sons, Inc., New York, N.Y., palladium has an electropotential of +0.987 volts, while copper has an electropotential of +0.337 volts.
One skilled in the art would expect palladium, being more noble, to be electrodeposited before copper. Similarly, one skilled in the art would expect tin, which has an electropotential of -0.136 volts, to deposit after, or perhaps together with, copper.
While electrodeposition techniques have been known for solution purification, and have been utilized for selective removal of metals, it has been used in instances where the more noble metals in the Electromotive Series are either electrodeposited first, or in some instances codeposited with other metals which are less noble.
With reference to U.S. Pat. No. 3,804,733, while copper has been removed from solutions also containing other metallic ions close to, or more noble than, copper in the Electromotive Series, such was not accomplished selectively, but rather by codeposition of the more noble metals including gold, silver, and platinum, which is closely related to palladium in the Electromotive Series. However, the present invention unexpectedly provides for selective removal of copper, in the presence of tin and the more noble metal palladium, which in view of the prior art knowledge would be expected to either codeposit or be deposited in the order of their ranking in the Electromotive Series.
Thus, the present invention provides a novel solution to the existing commercial need to purify, and thereby replenish, activator solutions containing palladium and tin, and does so by effecting selective removal of soluble copper contaminants.